Antennas capable of operating at multiple frequency bands are advantageous to many applications ranging from space-based radar to personal wireless communications. Synthetic aperture radar (SAR) typically operates in L- and C-bands. For space-based SAR applications where minimizing the mass and weight of the radar system is essential to reducing the overall cost of the mission, antennas capable of operating in multiple frequency bands with multiple polarizations are beneficial. Dual-band antenna elements are also desirable in radar applications because of their ability to improve data collection rates while also allowing for true multifunction radar (MFR) operation.
Wireless communications networks have shown an increased number of subscribers as well as an increased demand for multi-band equipment. Wireless access points and laptops are both turning towards antennas capable of operating in multiple frequency bands in order to support multiple protocol. The 2.4 GHz ISM band is quickly growing in popularity for wireless communications devices due to its use in Bluetooth technology and 802.11b/g protocol. For higher data rates, the frequency band from 5.15-5.85 GHz is often used, and the 802.11a protocol operates within the 5.2 GHz ISM band. Moreover, the cell phone industry is incorporating multi-band antennas into handsets to reduce the number of antennas required to provide operation for different services, e.g. as described in Bodley, M;: Sarcione, M.; Beltran. F.; Russell, M., “Dual band cellular antenna,” Wireless Applications Digest, 1997., IEEE MTT-S Symposium on Technologies. pp. 93-98, (February 1997).
Circular polarized (CP) antennas are popular choices in mobile wireless communications applications owing to their ability to allow flexible orientation between the transmitter and receiver antennas and to reduce multipath effects that can lead to signal fading. The ability to operate with both left hand (LH) and right hand (RH) senses of CP (LHCP and RHCP) allows the system to reuse frequencies and double the system capacity. In two-way data link systems, information is often transmitted by means of polarization shift keying, a technique that utilizes orthogonal senses of CP.
Dual-band and dual-polarized antennas have gained increasing popularity and have element architectures that can typically be placed into two categories: 1) a single element with a wide operational bandwidth capable of covering multiple bands or 2) an element comprised of two separate radiators, each of which is optimized for a specific frequency band. The majority of the work done on dual-band elements focuses on elements that operate with a single polarization state in each frequency band. There is some work that focuses on dual-band elements capable of supporting dual-linear operation at each band, and a minimal amount of work detailing dual-CP operation at each band. Moreover, much of the literature on dual-band operation details dual-band arrays using interleaved elements. In these designs, separate arrays of different sized elements are interleaved to form a single, dual-band aperture.
Microstrip patch antennas using the reactive stub loading has been shown to provide dual-band operation. However, each frequency band for this element operates with the same sense of linear polarization. If multiple feed locations and stubs are used, dual-linear polarization is possible. This type of elements has been shown to provide limited control of the frequency ratio between the two operational bands.
An annular ring patch radiator, e.g. as described in Cai. C.-H.; Row, J.-S.; Wong, K.-L., “Dual-frequency microstrip antenna with dual circular polarisation,” Electronics Letters, Vol. 42, no. 22, pp. 1261-1262 (October 2006), is capable of providing CP behavior at two separate frequency bands. When this type of element is operated in CP, the magnetic currents flow clockwise around the ring slot in a given frequency band, but they will flow counterclockwise at another frequency band. This behavior provides dual-band behavior, but each band only operates with a single sense of CP. There is also limited control over the ratio of frequencies for the two bands.
The cell phone industry has led to the design of several dual-band antennas. Duxian Liu; Gaucher, B., “A new multiband antenna for WLAN/cellular applications,” Vehicular Technology Conference, 2004. VTC2004-Fall. 2004 IEEE 60th, Vol. 1, pp. 243-246 (September 2004) describes a design capable of covering multiple frequency bands for cellular and WLAN applications. This element uses a combination of inverted-F and L-shaped radiators to cover the multiple bands. Lindmark, B. “A dual polarized dual band microstrip antenna for wireless communications.” Aerospace Conference, 1998. Proceedings., IEEE. Vol. 3. pp. 333-338 (March 1998) describes a dual-band antenna capable of covering GSM and DCS frequency bands consisting of an aperture coupled stacked patch design. Joo-Seong Jeon; Sang-Hoon Park, “Wideband antenna for PCS and IMT-2000 service band,” Vehicular Technology Conference, 2004. VTC2004-Fall. 2004 IEEE 60th. Vol. 1, pp. 216-219 (September 2004) describes a triangular shaped patch employing a U-shaped slot and L-shaped feed in order to provide a wide bandwidth capable of covering the PCS and IMT-2000 frequency bands. In each of these elements, the given frequency bands operates with only a single sense of linear polarization.
Many of the dual-band elements with CP polarization require complex feed networks consisting of diplexers and hybrids. U.S. Pat. No. 5,815,119, “Integrated Stacked Patch Antenna Polarizer Circularly Polarized Integrated Stacked Dual-Band Patch Antenna”, Helms et al., issued Sep. 29, 1998, is directed to a design for a dual-band stacked patch design where each band operates with a single sense of CP. In this design, the outputs of a 90° hybrid feed orthogonal locations on the element to generate CP. U.S. Pat. No. 6,114,997, “Low-Profile. Integrated Radiator Tiles for Wideband, Dual-Linear and Circular-Polarized Phased Array Applications”. Lee et al., issued Sep. 5, 2000, describes a wideband element capable of operating with linear, CP, dual-linear, or dual-CP polarization. The possible polarization states in this element depend on the configuration of a feed network consisting of 90° and 180° hybrids. U.S. Pat. No. 6,424,299, “Dual Hybrid-Fed Patch Element for Dual-Band Circular Polarization Radiation”, Cha et al., issued Jul. 23, 2002, describes a dual-band element with linear or CP operation with a hybrid feeding network.
Dual-band radiating apertures are often achieved by interleaving elements of different sizes, where each type of element has its own array lattice structure which in some designs is achieved by using perforated patches that enable a series of smaller elements to be placed within holes in the larger, low band elements. Although these purport to deal with dual-band apertures, the elements used in the design are inherently single band. The dual-band nature of the aperture stems from the arrangement of single band elements on different lattice structures.
There have been few attempts to design elements capable of simultaneously operating with orthogonal senses of CP. Jefferson, R. L.; Smith. D. “Dual circular polarised microstrip antenna design for a passive microwave transponder,” Antennas and Propagation, 1991. ICAP 91., Seventh International Conference on (IEE). Vol. 1. pp. 141-143 (April 1991) discloses a nearly square microstrip patch element utilizing orthogonal feed locations to simultaneously generate right hand CP (RI ICP) and left hand CP (LHCP). This element operates over a single frequency band.
It would therefore be desirable to provide an antenna having the capability to operate in two separate bands, with each band having the ability to simultaneously operate with dual-orthogonal polarizations (either dual-linear or dual-circular).